As an emission type electronic displaying device, there is an electroluminescent device (hereinafter referred to as ELD). As devices constituting the ELD, there are mentioned an inorganic electroluminescent device and an organic electroluminescent device (hereinafter referred to as organic EL device). The inorganic electroluminescent device has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the device. An organic EL device has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emission layer and recombined to form an exciton. The device emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the device can emit light by applying a relatively low voltage of from several volts to several decade volts. The device has a wide viewing angle and a high visuality since the device is of self light emission type Further, the device is a thin, complete solid device, and therefore, the device is noted from the viewpoint of space saving and portability.
However, development of an organic EL device for practical use is required which efficiently emits light with high luminance at a lower power.
High emission luminance and long lifetime is attained in Patent No. Japanese Patent No. 3093796 by doping a slight amount of a fluorescent compound in stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives.
A device is disclosed in Japanese Patent O.P.I. Publication No. 63-264692, which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound, and a device is disclosed in Japanese Patent O.P.I. Publication No. 3-255190, which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye.
When light emitted through excited singlet state is used as in the above, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%.
Since an organic EL device, employing phosphorescence through the excited triplet, was reported by Prinston University (see M. A. Baldo et al., Nature, 395, p 151-154 (1998)), study on materials emitting phosphorescence at room temperature has been actively made.
For example, such an organic EL device is disclosed in M. A. Baldo et al., Nature, 403, 17, p. 750-753 (2000) or U.S. Pat. No. 6,097,147.
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Such an organic EL device has possibility that exhibits the same performance as a cold cathode tube, and its application to illumination is watched.
Many compounds, mainly heavy metal complexes such as iridium complexes is synthesized and studied in for example, S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001).
An example employing tris(2-phenylpyridine)iridium as a dopant is studied in M. A. Baldo et al., Nature, 403, 17, p. 750-753 (2000) above.
Further, M. E. Tompson et. al. studies an example, employing as a dopant L2Ir (acac) such as (ppy)2Ir (acac) in The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu), and Moon-Jae Youn. Og, Tetsuo Tsutsui et. al. an example employing as a dopant tris(2-p-tolylpyridine)iridium {Ir(ptpy)3} or tris(benzo-[h]-quinoline)iridium {Ir(bzq)3} in The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu). (These metal complexes are generally called orthometalated iridium complexes.)
Attempt for preparing a device employing various iridium complexes is made in S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001) above.
Orthometalated complexes in which iridium as a center metal is replaced with platinum are also watched. Regarding these complexes, there are known many kinds of complexes having characteristics in the ligands (see for example, Patent Document Nos. 1 through 5 below)
As host compounds of these phosphorescence emitting dopants, carbazole derivatives represented by CBP or m-CP are well known. There are well known m-CP or its derivatives as a host emitting a blue light, however, these compounds do not provide satisfactory efficiency or lifetime (see for example, Patent Document Nos. 6 and 7 below),    Patent document 1: Japanese Patent O.P.I. Publication No. 2002-332291    Patent document 2: Japanese Patent O.P.I. Publication No. 2002-332292    Patent document 3: Japanese Patent O.P.I. Publication No 2002-338588    Patent document 4: Japanese Patent O.P.I. Publication No. 2002-226495    Patent document 5: Japanese Patent O.P.I. Publication No. 2002-234894    Patent document 6: WO 03/080760    Patent document 7: WO 04/074399